Raise GitProtocolError on unexpected pkt

There are some packets (like NAK) which have no sha. This does not change them to be handled but you get in exception always the command that failed to be unpacked instead of strange exception about there not being enough values to unpack.

Raise GitProtocolError when encountering HTTP Errors in HTTPGitClient

Fixes #1199

More refactoring of index

Define enum for stage values

config: check both `git/etc` and `git/mingw64/etc` on windows

This should make the changes transparent to existing API users (so long as they don't work in trees with conflicts). It also prevents repeated dictionary access.

client: Fix content type check in AbstractHttpGitClient._smart_request

FIx iterobjects_subset

Some git servers can send a Content-Type header containing a charset directive. Official git client can successfully clone repositories hosted on such server but dulwich was raising a GitProtocolError as the code to check the header value was not expecting a directive to be present in it.

Add the readthedocs configuration

Fixes #1166

Handle missing object in iterobjects_subset